A fuel cell system is known which again supplies, to a fuel cell, a fuel off gas discharged from the fuel cell with a circulation pump (e.g., see Japanese Patent Application Laid-Open Nos. 2003-178782 and 2004-22198). The fuel off gas includes generated water or has a high humidity. Therefore, in a low-temperature atmosphere after the system stops, the generated water which might remain in the circulation pump or condensed moisture is sometimes frozen. When a torque is generated in the circulation pump having a frozen state during restart of the system, an impeller of the circulation pump might be broken, and the circulation pump might thus be damaged. To prevent the freezing of the circulation pump, in the fuel cell system disclosed in Japanese Patent Application Laid-Open No. 2003-178782, during the system stop, a dry fuel gas is introduced into the circulation pump. The fuel off gas in the circulation pump is replaced with the dry fuel gas, whereby the fuel cell system performs scavenging processing.